Trim down test_multiple_paths.py

Make all all tested entities (i.e. classes in entities.py) inherit from
`EntityReadMixin`. Take advantage of this fact to trim down module
`test_multiple_paths`.

Add a logger to module `test_multiple_paths`. Log out information in several
tests, thus allowing the assertions to be trimmed down.
Merge pull request #1343 from Ichimonji10/test-multiple-paths

Trim down test_multiple_paths.py
Monkey patch ddt.data when generating docs

This will make Sphynx generate better docs by not add an entry for each
created test by DDT.